Precision wire wound resistors



v- 22, 1955 J. DANIELS 2,724,759

PRECISION WIRE WOUND RESISTORS Filed March 4, 1954 2 Sheets-Sheet 1FuJ/b/e Mefa/Sfr/p ,4 (0077/79 3 INVENTOR.

1'' BY a) M 57 L ATTORNEYS Nov. 22, 1955 J. L. DANIELS 2,724,759

PRECISION WIRE WOUND RESISTORS Filed March 4, 1954 2 Sheets-Sheet 2Joseph L.D n' s BY MSW it ATTORNEYS United States Patent PRECISION WIREWOUND RESISTORS Joseph L. Daniels, Levittown, N. Y., assignor toVari-Ohm Corp., Amityville, N. Y., a corporation of New York ApplicationMarch 4, 1954, Serial No. 414,097

17 Claims. (Cl. 201-48) The present invention relates to wire woundprecision resistors and to apparatus utilizing such resistors.

Inaccuracies which arise from slight variations in the resistivity anddiameter of the resistance wire, variations in the dimensions of theforms upon which the wire is wound, and other factors make itcommercially impractical to wind wire resistors to a fixed number ofturns on commercially produced forms, if a precision appreciably greaterthan :l% is desired.

The manufacture of resistors of the wire wound type to achieve a higherdegree of precision than :l% has heretofore involved a final adjustmentby the careful removal of excess resistance wire, the excess wire beingpurposely included in the resistor in order to provide a range ofadjustability.

An object of the present invention is to provide means for adjusting awire wound resistor to its correct nominal value with an extremely highdegree of precision, such adjustment being made after assembly of thecompleted resistor and without the necessity for winding or unwindingany resistance wire.

A further object of the invention is' to provide means for the preciseadjustment of the resistance value of a relatively inaccurate resistorwound in accordance with conventional production methods.

A further object of the invention is the provision of a variableresistor, potentiometer, or similar apparatus in which the relationshipbetween the angular position of a shaft and the resistance valuesincluded between certain terminals of the apparatus is determined withan unusually high degree of precision.

Still another object of the invention is to provide a potentiometer;variable resistor, or similar device in which a resistance value isaccurately related to the angular position of a rotatable shaft, theaccuracy being obtained by the adjustment of a vernier resistor, suchadjustment being made' by increasing the resistance value of the vernierresistor, the vernier resistor being added to the basic adjustableresistor structure after a preliminary approximate adjustment of thebasic resistor structure.

"A feature of the invention resides in the fact that adjustability isobtained without the use of movable taps.

Other and further objects and advantages of the invention will becomeapparent upon reading the following specification together with theaccompanyingdrawing forming a part hereof. 7

Referring to the drawing: I

Figure l is an enlarged diagrammatic view showing means for permanentlyadjusting a wire wound" resistor without the winding or unwinding of anyresistance wire.

Figure 2 is a transverse sectional view taken along the line 2-2 ofFigure 1, looking in the direction of the arrows.

Figure 3 illustrates means for obtaining a more precise adjustment thanis obtainable using the arrangement of Fig. 1.

Figure 4 illustrates an arrangement for the precise adice justment of arelatively inaccurate commercial resistor by means of a small vernierresistor of relatively low resistance.

Figure 5 is a circuit diagram of a precision potentiometer using thearrangements illustrated in Figs. 1 to 3.

Figure 6 is a rear perspective view of the potentiometer shown in Fig.5, the potentiometer being broken away to illustrate details ofconstruction.

Referring to Figs. 1 and 2, a resistance wire 10 is shown helicallywound on a core 11 of suitable insulating material. At the left handportion of core 11, a strip of electrically conductive fusible metal 12is disposed in electrical contact with a plurality of adjacent turns ofresistance wire 10 under the convolutions 13 of the resistance wire 10,effectively short circuiting these turns and making the total resistanceof the entire winding substantially equal to the resistance of theconvolutions 14 of the right hand end of the resistor beyond the righthand end 15 of the fusible metal strip 12.

The fusible metal strip 12 is formed of metal having a melting pointlower than the melting point of the resistance wire 10. Advantageously,a corrosion resistant metal such as gold or a gold alloy may be used forthe metal strip 12, in order that the electrical contact between themetal strip 12 and the convolutions 13 of resistance wire 10 may remainpermanent, constant, and unaffected by atmospheric conditions. The widthand the thickness of the fusible metal strip 12 will be determined bythe maximum current it is required to carry. Where the maximum currentis very small, gold leaf or its equivalent may be used.

The finished resistor is initially manufactured and arranged to have asmaller resistance than the value desired. In order to increase theresistance to the correct value, current is caused to flow through theleft hand portion 13 of the resistor in which the fusible metal strip 12is located. By means of suitable contacts (not shown) the current may beconfined to any'desired portion of the resistor. By the use of a verylow voltage to produce the current flow, the current will drop as soonas a single turn or single additional turn has been unshunted by themelting of the fusible metal strip 12 between two adjacent convolutionsof the resistance wire 10. Experience will indicate with certainty howmany additional turns of resistance wire are required to bring theresistance to the correct nominal value, and the metal strip 12 may thenbe fused accordingly so that the shunting action of strip 12 iseliminated for the correct number of turns.

From the foregoing, it will be seen that the adjustment may be made tobring the resistance value of the resistor incrementally to the nearestresistance value attainable by the inclusion or omission of a full turnof resistance wire 10. Where the turn resistance is small compared withthe limits of tolerance, satisfactory results may be achieved using asingle resistor as shown in Fig. 1.

Where a greater precision is required, the arrangement shown in Fig. 3is utilized. In Fig. 3, a fixed resistor 20 is connected in multiplewith an adjustable or vernier resistor .21 the vernier resistor 21having a fusible metal strip 22 similar to the strip 12 of Figs. 1 and 2which may be used for progressively increasing its resistance. Let it beassumed that the adjustable resistor 21 has. aresistance value of x andthat the fixed resistor 20 has a resistance value of y, the resistors 20and 21 being connected in parallel to terminals 23 to produce a combinedresistance value of R. According to the conventional formula for tworesistors connected in parallel:

Let it be assumed that the resistance x of the adjustable resistor 21 ismade several times greater than the resistance value y of the fixedresistor 20, such that:

a; F'y, or y 5 Then substituting into the expression the followingexpression is obtained in which y has been From this last expression, itwill be seen that if F is made fairly large, say 99, so that theresistance value x of resistor 21 is 99 times the resistance value y ofresistor 20, then dR/a'x becomes A This means that a 1% change in thevalue of x will produce a of 1% change in the total resistance value Rmeasured at terminals 23.

If a final precision of of 1% is desired for the value of R, then avalue of 19 for F is required if the value of x is to be adjusted onlyto the nearest 1%. These figures are merely illustrative and serve toshow that by proper proportioning of the resistance values and theprecisions of the resistance values y and x of the resistors 20 and 21,respectively, that by adjustment of the resistance value x of thevernier resistor 21 to the nearest turn, a much higher degree ofprecision is obtainable in the combined resistance value R than in theprecision of adjustment of the vernier resistance value x.

It should be further noted, that both resistors 20 and 21 may both bemade adjustable by means of a fusible metal strip 22 in each resistor,the lower resistance value y of resistor 20 being first adjusted to thenearest turn, 0 whereafter the resistance value x of resistor 21 isadjusted to the nearest turn. Where a greater degree of precision isdesired, three or more resistors of progressively increasing resistancevalues may be connected in multiple, and the first adjustment may thenbe made by adjusting the resistor of lowest resistance to the nearestturn so that the combined resistance R is just under the desired finalvalue. The resistor of next higher resistance value is then adjusted sothat the value of R is still just under the desired value but with agreater degree of precision than in the original adjustment. This maythen be followed by an adjustment of the third resistor, etc. eachsuccessive adjustment increasing the combined resistance value R bysuccessively smaller increments until the desired resistance value for Rhas been attained a with the desired degree of precision.

The resistance wire 20 may be formed of any suitable material having dueregard for its resistivity and temperature eoefiicient of resistance.

The fusible metal strip 12 in Fig. l or the fusible metal strip 22 inFig. 2, is preferably formed of a suitable metal or other electricallyconductive material having a melting point lower than that of theresistance wire 10. In this manner, resistance wire 10 may carry acurrent sufiicient to fuse the metal strip 12 or 22 before theresistance wire might become fused. By the use of suitable contactmembers (not shown), lateral contact may be made with adjacent turns ofresistance wire 20 to fuse the metal strip therebetween without passingcurrent longitudinally along the resistance wire, in which case the heatdissipation factors may be so arranged that the relative melting pointsof the resistance wire and of the fusible metal strip are ofcomparatively minor importance.

As an example, the metal strip 12 may be formed of a gold-copper alloyhaving a melting point in the range from 900950 C. such as an alloycontaining Au 92%, the balance being Cu. The resistance wire 20 may be anickel-chromium alloy containing Ni Cr. 20%, sold under the commercialdesignation Nichrome, and having a melting point in the range l300-l400C.

Figure 4 illustrates the adjustment of a commercial resistor of lowprecision to a precise value by means of a small vernier resistor ofrelatively low resistance. The fixed resistor 20 is produced accordingto any desired conventional method, its precision being of relativelylittle importance. The fixed resistor 20 is provided with terminals 24and 25 and is tapped at 26 near terminal 24. A relatively smallresistance y, is included between terminal 24 and tap point 26. Arelatively large resistance Y, is included between tap point 26 andterminal 25.

The vernier resistor 21 is wire wound and is provided with terminals 27and 28. Initially, the fusible strip 22 may extend throughout the entirelength of the vernier resistor 21. The adjustable resistance value x ofthe vernier resistor is connected in multiple with the small fixedresistance value y of the fixed resistor 20. The fixed resistor 20 willalways have a total resistance value y-j-Y greater than the desiredfinal precise resistance value R measured between terminals 23. Theresistance value y is shunted by the adjustable vernier resistance valuex and will initially bring the total resistance value R to a value lessthan the desired precise value. By progressively fusing the fusiblemetal strip 22, the final total resistance value R may be adjusted witha high degree of precision. Since only a small portion of the fixedresistor is included between terminal 24 and tap point 26 and this smallportion is being adjusted by the vernier resistor 21, a greater degreeof precision is obtained in the total resistance R than in the precisionof the parallel combination of x and y.

For example, if the value of resistance R is to be 1000 ohmsi of 1% andthe value of y between terminal 24 and tap point 26 is 50 ohms, thetotal resistance Y+y between terminals 24 and 25 may be wound to 1025ohms:2% bringing the minimum value of total resistance Y+y to about 1005ohms and the maximum value of total resistance Y+y to 1045 ohms with yunshunted.

With a zero value for x, these limits will be 955 to 995 ohms. Byprogressively increasing the value of x, a total value of 1000 ohms maybe obtained under all circumstances. Since a final precision of of 1% isdesired, the total resistance value must be adjusted to the nearest A2ohm. For the 50 ohm section between terminal 24 and tap point 26, aprecision of /2 ohm is a precision of only 1%. The vernier resistor 21thus requires fewer than turns, since a shunt adjustment is involved, asdiscussed above in connection with Fig. 3.

Figures 4 and 5 show a precision potentiometer which embodies theinvention to obtain precise resistance values which are accuratelyadjusted and linearly interpolated for predetermined angular positionsof the potentiometer control and adjustment shaft. This permits the useof accurately pre-calibrated dials with different potentiometers,thereby avoiding the necessity for individually calibrating each dialfor each individual potentiometer. In many instances, where thepotentiometer might be controlled by complex mechanism, thepotentiometer must necessarily be fitted to follow the angularadjustments imparted thereto by the complex mechanism, it

being virtually impossible to adjust such a complex mechanism to theindividual angle-resistance characteristics of the potentiometer.

The potentiometer shown in Figs. and 6 comprises a body 30 provided witha rotatable shaft 31 which is shown journaled in the body 30 by means offront and rear ball bearings 32 and 33. A disc of insulating material 34is fixed to the rear end of shaft 31 for rotation therewith. A rotarycontact arm 35 is fixed to disc 34 by screws 36. Connection with rotarycontact arm 35 is established by any desired conventional means, such asa flexible pigtail (not shown) the details of such means being omittedfor simplicity of illustration.

A collar 37 is fixed to shaft 31 intermediate the rear ball bearing 33and the insulating disc 34, the collar 37 having a circumferentialgroove 38 formed therein. A resilient friction washer 39 is providedwith parallel transverse ribs 40 which engage diametrically opposedportions of collar 37, passing through the circumferem tial groove 38. Abent wire 41 engages one side of friction washer 39 and comprises astraight portion 42 which is longitudinally slidable in body 30,extending therethrough in a direction parallel to shaft 31 andterminating in a free end portion 43. By pressing against the free endportion 43 of bent wire 41, the pressure of engagement betweentransverse ribs 40 of friction washer 39 and collar 37 may be varied toobtain an adjustable frictional drag for holding rotatable shaft 31 inany desired position of angular adjustment, together with the contactarm 35 which rotates therewith.

At its free end, contact arm 35 carries a resilient contact member 44which is fixedly secured thereto at 45 as by welding, brazing or thelike. The contact pressure of contact member 44 may be adjusted by meansof an adjusting screw 46 threaded into a bent end portion 47 of contactarm 35.

Contact member 44 is in sliding engagement with a circular wire woundcontact resistor 48 wound on a core 49. The resistance of contactresistor 48 is substantially uniform for each unit length throughout itsentire length. The contact resistor 48 is laterally confined between twoadjacent annular members 50 having peripherally spaced aligned radialV-shaped notches 51 formed therein. A plurality of axially spacedannular members 50 are carried by body 30, the notches 51 on all of theannular members 50 being shown in axial alignment with respect to shaft31.

Extending transversely across annular members 50 through the alignedV-shaped notches 51 are a plurality of bridging wires 52 which engagethe internal periphery of contact resistor 48 at spaced points along itslength.

Disposed between further annular members 50 forwardly of and adjacent tocontac resistor 48 is a function resistor 54. More forwardly disposedbetween further annular members 50 and adjacent to function resistor 54is a Vernier resistor 55. Each of the bridging wires 52 engages spacedpoints along the internal peripheries of all three resistors 48, 54 and55, connecting these points in multiple. Resilient ring members 56formed of stable plastic resilient material such as synthetic rubber,yieldingly press the bridging wires 52 into constant contact with innerperipheries of the three resistors 48, 54 and 55. Bridging wires 52 neednot necessarily be straight, but may be bent to skip one or morenotches, if desired, so that a greater portion of one resistor isincluded between adjacent bridging wires than of another resistor. Thisis indicated diagrammatically at 52a in Fig. 5.

In order to provide mechanical stability over a wide range of ambienttemperature variations, the core 49 of contact resistor 48 and of theother resistors 54 and 55 is formed of metal having the same thermalcoefficients of expansion as the resistance wire which is wound thereon,the core material being coated with a suitable insulating material. Inorder to obtain this condition with accuracy, the core is preferablyformed of resistance wire material of the same composition as thecurrent carrying resistance wire. The core may also be formed of hollowtubing, if desired.

The function resistor 54 and the Vernier resistor 55 are each providedwith suitable fusible metal strips 12 as described for the singleresistor shown in Figs. 1 and 2, a fusible metal strip 12 being situatedbetween each two points of contact of each resistor with the bridgingwires 52, or a fusible metal strip 12 may initially extend throughoutthe entire length of each resistor and may thereafter be subsequentlyfused as desired.

The resistance of the function resistor 54 is adjusted after partialassembly of the potentiometer without the Vernier resistor to provideapproximately predfetermined resistance values between each two adjacentbridging wires 52 to conform to incremental values of some desiredfunction of resistance with respect to the angular position of shaft 31.Between adjacent bridging wires, the resistance variation with respectto angular shaft position will necessarily be linear. However, bychanging the resistance between adjacent bridging Wires to correspondapproximately to the slope or first derivative of the curve of thedesired function between these adjacent points a first approximation maybe obtained. After finalaccurate adjustment, the resistance valueintermediate these positions is the equivalent of interpolation in thecase of mathematical tables. After adjustmerit of the function resistor54 has been completed to obtain slightly more than the desiredresistance value between any two bridging wires, the Vernier resistor isapplied to the assembly and the resistance between the same two pointsis then precisely adjusted to bring the combined resistance to thedesired value with a high degree of precision between adjacent bridgingwires 52 as previously described. In this connection, the resistance perunit length of the Vernier resistor Will preferably be considerablygreater than that of the function resistor.

A fixed resistor 57 may conveniently be mounted as shown. The fixedresistor 57, however, is ordinarily not engaged by the bridging wires52. The fixed resistor 57 may also comprise an adjustable portioncontaining a fusible metal strip, if desired. I

It is to be understood that the potentiometer construction illustratedmay alternatively embody resistors using a core having an insulatingsurface coated with resistive metal which is removed from the surface asrequired to adjust the resistance between adjacent bridging wiresto therequired value.

Many changes in details of construction will be apparent to thoseskilled in the art, and such changes may be made without departing fromthe scope of the invention as defined in the following claims.

What is claimed is:

1. The method of incrementally increasing the resistance of a fixedresistor comprising a plurality of convolutions of a resistance wire anda fusible electrically conductive member in electrical contact with aplurality of adjacent turns, said method comprising the step ofprogressively fusing said fusible member.

2. The method of obtaining a resistor. having a desired resistance valueto a high degree of precision, said method comprising the steps of:shunting at least a portion of said resistorv with a further resistor,and adjusting the resistance of said further resistor, whereby theprecision of the combined resistances of said resistor and said furtherresistor is greater than the precision of adjustment of said furtherresistor.

3. The method of incrementally increasing the combined resistance of aplurality of fixed resistors connected in parallel, one of saidresistors comprising a plurality of convolutions of resistance wire anda fusible electrically conductive member in electrical contact with aplurality of adjacent turns, said method comprising the step ofprogressively fusing said fusible member, whereby said combinedresistance is progressively incrementally increased by successiveincrements each of which is less than the resistance of a singleconvolution the resistance of which has been caused to become eifectiveby said fusing of said fusible member.

4. A resistor of which the resistance may be incrementally increased,comprising: a plurality of convolutions of resistance wire; and anelectrically conductive fusible member in electrical contact with aplurality of adjacent convolutions of said resistance wire, whereby thefusing of said fusible member between two adjacent convolutions willincrease the resistance of said resistor by the resistance of one ofsaid convolutions.

5. A resistor according to claim 4, in which said fusible member isformed of a gold alloy.

6. A resistor according to claim 4, in which the melting point of saidfusible member is lower than the melting point of said resistance wire,whereby said fusible member may be fused by the passage of an electricalcurrent through said resistance wire.

7. A resistor according to claim 4, further comprising an insulativecore upon which said resistance wire is wound, said fusible member beingin the form of a thin strip disposed between at least a portion of saidturns of resistance wire and said core.

8. A resistor comprising a core formed of resistance wire material; aninsulating coating on said core; and a winding of resistance wire of thesame composition as said core wound upon said insulating coating,whereby dimensional changes in said core accompanying changes in ambienttemperature will conform to the corresponding dimensional changes insaid resistance wire.

9. A potentiometer of the class described, comprising: in combinationwith a body; a shaft rotatably disposed in said body; a contact armcarried by said shaft for rotation therewith; a circular resistorconcentric with said shaft and carried by said body; a resilient contactmember carried by said arm and engageable with different longitudinalportions of said resistor, the provision of a further resistorelectrically connected to said first named resistor at spaced pointsalong both of said resistors, said further resistor comprising means foradjusting the resistance thereof to obtain a combined resistance betweensaid spaced points whose precision is greater than the precision of saidadjustment.

10. A potentiometer according to claim 9, in which said adjustableresistor comprises a core having an insulating surface coated with aresistive metal which may be removed as required to increase theresistance between said spaced points.

11. A potentiometer according to claim 9, in which said connection atsaid spaced points is eifectedby bridging wires extending between saidresistors, said potentiometer further comprising resilient means urgingsaid bridging wires into contact with said resistors.

12. A potentiometer according to claim 9, in which both of saidresistors are of circular configuration of equal radius and concentricwith said shaft, said resistors being axially spaced from each other.

13. A potentiometer according to claim 9, wherein said bridging wiresextend substantially parallel to said shaft.

14. A potentiometer of the class described, comprising: in combinationwith a body; a shaft rotatably disposed in said body; a contact armcarried by said shaft for rotation therewith; a circular resistorconcentric with said shaft and carried by said body; a resilient contactmember carried by said arm and engageable with different longitudinalportions of said resistor, the provision of a further resistorelectrically connected to said first named resistor at spaced pointsalong both of said resistors, said further resistor comprising aplurality of turns of resistance wire and an electrically conductivefusible member in electrical contact with a plurality of adjacent turnsof said resistance wire intermediate at least two of said spaced points,whereby the resistance of said first named resistor intermediate saidtwo spaced points may be incrementally adjusted by fusing of a portionof said fusible member intermediate adjacent turns of said resistancewire with which said fusible member is in contact.

15. A potentiometer according to claim 14, in which said connection atsaid spaced points is effected by bridging wires extending between saidresistors, said potentiometer further comprising resilient means urgingsaid bridging wires into contact with said resistors.

16. A potentiometer according to claim 14, in which both of saidresistors are of circular configuration of equal radius and concentricwith said shaft, said resistors being axially spaced from each other.

17. A potentiometer according to claim 14, wherein said bridging wiresextend substantially parallel to said shaft.

References (Iited in the file of this patent UNITED STATES PATENTS

